Transfer and adaptation of an image or images stored on a medium is used in a variety of commercial and consumer activities. The image adaptation is designed so that the observer of a target display system perceives the image in a desired way, and most typically so that the observer perceives the image in a way that is most true to the original source of the image or that is most true to the way it would have been perceived on an original display system to which the originally stored image had been adapted.
Today, images are captured by a variety of means such as digital cameras and digital scanners. Digital cameras provide data that can be viewed on display devices such as Cathode Ray Tubes (CRT) monitors or printed onto a variety of media, most commonly paper. Digital scanners can take printed or film images and create digital data that can be displayed or reproduced onto printed media. Movies are commonly converted from film or digital data to various media for use in consumer television. Digital data can be computer-generated or generated by specially designed digital devices.
A display system recreates the complex colors and intensities for the observer. Specific media and their corresponding display systems have specific spectrum characteristics. Film has a continuous color spectrum that reproduces the image by absorbing certain colors and reflecting others. This type of display system is known as a subtractive color synthesis system. Other display systems are additive color synthesis systems because they use a light source to create and combine the colors according to their respective intensities. Additive color systems include CRTs, Liquid Crystal Displays (LCDs), Digital Micromirror Device (“DMD”)-based displays, and plasma displays. These additive color systems reproduce an image by using a mixture of primary colors. The CRT and plasma displays use phosphors, which have self-luminance properties, whereas other displays such as DMDs sometimes project mixtures of white light that has been filtered down to the three primary color components. In most cases, it is desirable that the observers' perception of the reproduced image correlate to the color spectrum of the original image that is being reproduced. None of the color systems can exactly reproduce the full color range of the human eye.
Red, green and blue are used in additive systems because they combine to match the light spectra that are most readily perceived by the human eye. In a phosphor-based system, for example, the available colors of the phosphors create a limited color gamut that can be defined by a chromaticity diagram, which will be discussed below. Other additive systems have similar limitations. For instance the light of projection systems is often filtered from a white light source, and those filters and the original light source have limited spectral bands, whereby a limited color gamut also describes the capabilities of the white-light-based projection system.
A trade-off exists between the quality or “hue” of a color and its purity or “saturation” by white light. If a given set of color filters maintains a very pure color, then the brightness will be lower. Relaxing the purity and increasing the white light will provide a brighter image but may produce undesired color tints.
Converting film into other media involves many different parameters. When a movie is produced, the frame rate is conventionally 24 frames per second (fps). Correspondingly, the NTSC standard for television frames is 29.97 fps. A process known as telecine is used to convert from the “film” standard to the “video” standard. This video media could be in analog or digital form and written to Video Cassette Recorder (VCR) magnetic tape, Digital Video Disk (DVD), and computer storage medium. In the telecine process, several conversions and adjustments take place. A common telecine system converts the film media into a digital image using a Charged Coupled Device (CCD). The resolution of the image may vary but the result is that the image is now represented by a matrix of pixels. As noted, the frame rate must be adjusted. Extra frames are inserted into the output to maintain the same time sequencing. This adjustment is typically not done for DVD or PAL systems because the DVD systems typically have the capability to adjust the frame rate and the PAL system frame rate at 25 fps is more consistent with the 24 fps film or cinema standard.
The conversion of film to other media further uses a human observer, known as a “colorist,” who views the content and uses equipment that is interfaced with or included in the telecine to make any color corrections based on the colorist's judgment. CRT monitors are typically used by colorists to monitor and adjust the scene as needed. A challenge in the field is posed by the variation of CRT color performance, both between different CRTs and for performance variations of a single CRT as it ages. Because of these variations, calibration of the CRTs is needed to maintain consistent performance. Further, the color gamut available through CRT monitors is less than the color gamut of newer-technology display systems on which the film to be converted may later be displayed. One such example of a newer technology, discussed later, that uses a DMD device is a Digital Light Projector (DLP) system.
The process is challenging when converting from the subtractive color system of film to the additive color systems like CRT, LCD, and DMD displays. Movies are a continuous stream of varying images, which demands close attention by the colorist, who is the human observer that adjusts the system parameters during the image conversion. For the purpose of this disclosure, the film process will be described, however, other dynamic systems can use the same techniques and systems that are disclosed below, and further, the techniques and systems described below might be applied in the context of static image adaptation.